1. Field of the Invention
The present invention relates to a digital still camera. In particular, the present invention relates to a digital still camera that can read high quality image data from an image pickup device in both a low sensitivity mode and a high sensitivity mode.
2. Description of the Related Art
Among the digital still cameras having a mounted image pickup device such as a CCD, there are some for which sensitivity can be altered. One example digital still camera for which film sensitivity can be set has a low sensitivity mode, used for shooting at a low sensitivity corresponding to an ISO standard of from 100 to 400, and a high sensitivity mode, used for shooting at a high sensitivity corresponding to an ISO standard equal to or greater than 800. In the low sensitivity mode, the amount of the signal charges accumulated by each imaging pixel (photodiode) is increased because the period the shutter is open is extended. While in the high sensitivity mode, the amount of the signal charges accumulated by each imaging pixel is reduced because the period the shutter is open is reduced.
The signal charges accumulated by the imaging pixels are read and transmitted along a vertical transfer path. FIG. 8 is a graph showing a transient change in the amount of the signal charges, i.e., the number of electrons accumulated by the individual imaging pixels. After the shutter is closed, the amount of the signal charges is exponentially reduced by thermal diffusion as time elapses. Therefore, if the signal charges are not immediately read from the image pickup device, the quality of the produced image will be degraded.
The signal charges read from the imaging pixels and transmitted via the vertical transfer path pass through potential wells sequentially formed along the vertical transfer path. Specifically, however, each time charges are transmitted tiny numbers of electrons remain in the potential wells. As a result, these electrons are mixed with signal charges read from imaging pixels for the next colors and are transmitted along the vertical transfer path. This is another factor that contributes to the degradation of the image quality.
In the high sensitivity mode, the individual imaging pixels accumulate only a small amount of signal charges. When these signal charges are gradually reduced while being transmitted along the vertical transfer path, the signal charges are mixed with other signal charges for other colors. Therefore, because the original signal charges are small, the effect produced by color mixing is increased and the image quality is degraded. However, as is apparent from FIG. 8, when the signal charges are small (the foot portion of the characteristic line in FIG. 8), likewise, the rate at which the number of electrons is reduced as time elapses is small, so that the effect is reduced, even when it takes time to read signal charges from the image pickup device.
In the low sensitivity mode, a large amount of signal charges are accumulated in the individual imaging pixels. Even when a specifically tiny number of electrons remains in each potential well along the vertical transfer path, and is mixed with signal charges for other colors, image deterioration due to color mixing is so small it can be ignored because the original signal charges are large. However, as is apparent from FIG. 8, when the signal charges are large, there is an increased reduction in the number of electrons as time elapses, and this phenomenon can not be ignored as a factor that can contribute to image deterioration.
As described above, the problems encountered when photographing in the high sensitivity mode or in the low sensitivity mode are the direct opposites of those encountered in the opposing mode, and if both sets of problems are not resolved, the image quality in the two modes can not be improved.